CN110997164A

CN110997164A - Powder coating method and coated article

Info

Publication number: CN110997164A
Application number: CN201880053735.7A
Authority: CN
Inventors: M·D·麦克科米克; O·艾哈拉-马里蒙; F·齐默曼
Original assignee: Stahl International BV
Current assignee: Akzo Nobel Coatings International BV
Priority date: 2017-07-07
Filing date: 2018-07-06
Publication date: 2020-04-10
Also published as: WO2019009723A1; CA3068807A1; BR112020000230A2; EP3648901A1; US11865574B2; CN117600038A; EP3648901B1; MX2020000161A; RU2020103558A; ZA202000283B; US20200139400A1; NL2019197B1

Abstract

The present invention relates to a method of coating an article and a coated article. One method according to the invention comprises: providing a substrate coated with a first powder coating layer; optionally heating the substrate and first layer to at least partially melt or soften the first powder coating layer, wherein said first powder coating layer is uncured or only partially cured; applying a second powder coating layer onto the first layer; and curing the first and second layers.

Description

Powder coating method and coated article

The present invention relates to powder coating technology and in particular to a method of powder coating an article; and to a coated article obtained with such a method. In more detail, an embodiment of a method for powder coating a heat sensitive substrate (e.g., MDF or wood) with a powder coating composition is provided. The invention allows in particular to obtain a desired smooth low-gloss finish (finish).

The invention uses a method in which two powder coating layers are applied.

Some prior art documents also describe powder coating methods using two powder coating layers. However, these documents do not teach how to achieve a smooth low gloss finish, especially on heat sensitive substrates. Herein, "smooth" refers to how the coating feels, and low gloss refers to visual appearance. The coating may also be smooth looking, i.e. have a uniform appearance. The smooth finish has minimal or even no orange finish.

US2004/0253373 describes a method of coating a non-conductive plastic substrate comprising the steps of: (a) cleaning the substrate; (b) applying an adhesive/sealant to the substrate; (c) curing the adhesive/sealant by means of heat; (d) applying a thermosetting powder to the hot substrate; and (e) curing the thermosetting powder with heat. The method leaves room for improvement of e.g. the obtained texture.

US2007/0224352 describes a method wherein the substrate is coated and wherein for heat sensitive substrates such as MDF, the first coating is green cured just prior to applying the second powder coating. The method uses pulsed heating with a plurality of fixed infrared radiant heat sources. The document is silent about the gloss of the coating.

US2014/0127417 describes a process in which MDF boards are electrostatically coated with epoxy powder and then conveyed through an infrared gel oven to cause the epoxy powder to gel or partially liquefy. The gelled MDF board was then transported in about 8 minutes to a top coating chamber where the board was coated with another powder. The panel is thereafter conveyed to a curing oven where the primer coating and topcoat are cured. This document does not describe how to obtain smooth low gloss coatings.

Conventional powder coating compositions have a softening temperature (Tg or glass transition temperature) of, for example, about 40-60 ℃, and a melting temperature (Tm) of about 75-110 ℃, where Tm refers to the incipient melting (initiation) temperature. Such compositions may be cured, for example, at a temperature of about 200 ℃ for at least 10 minutes. Typically, the low bake composition cures at lower temperatures, such as 135-150 ℃ for 3-5 minutes. Conventional powder coatings are not suitable for wood-based substrates because these substrates are heat sensitive and are prone to, for example, bending, cracking and outgassing at conventional curing temperatures, which can cause, for example, poor uniformity of the coating layer. Furthermore, fiber erection (fibre rise) from wood-based substrates such as MDF, solid wood, and thin wood board parts necessitates pre-and/or post-treatments such as priming and sandblasting before the final overcoat can be applied or in order to give a single coating with good surface quality.

EP1092479 describes a heat-sensitive substrate which is coated with a first low-temperature-curing coating powder and a second high-temperature-curing coating powder. If a high temperature curing coating powder is applied, fused and cured directly on a substrate, the time/temperature conditions for fusing and curing the high temperature coating powder will damage the substrate. However, when the high temperature curing coating powder fuses and cures on the substrate, the coating provided by the low temperature curing coating powder protects the substrate from damage.

Matte additives like waxes are also known, but are generally not suitable for curing at low temperatures, for example at less than 130 ℃. Such additives can also adversely affect mechanical properties, orange peel, scratch resistance, yellowing, brittleness, fingerprint marking, and chemical resistance. Other texture additives include PTFE-based additives or additives containing particulates.

In general, smooth finishes are difficult to achieve with low temperature curing powder coating compositions. In view of the generally high reactivity of such coating compositions, powder coating compositions generally have a short time after melting and then begin to cure. What is desired is a method of achieving a smooth, low gloss finish, and more desirably with minimal orange peel. Desirably, the overlay is non-textured, i.e., smooth to the touch. Desirably, the method is suitable for powder coating for heat sensitive substrates. This smooth, low gloss finish may be referred to as satin smooth.

It is an object of the present invention to provide a powder coating process which allows a flexible combination of texture and gloss.

It is another object to provide a coated article having a smooth, low gloss finish. Desirably, the gloss is uniform across the coated article, preferably a uniform low gloss across the coated surface of the coated article. Furthermore, the coating is desirably suitable for heat sensitive substrates and/or substrates comprising lignocellulosic materials, such as substrates comprising or based on wood, more particularly MDF.

It has surprisingly been found that these objectives can be at least partially met in a two-step process, wherein a second coating layer is applied onto the uncured or incompletely cured first coating layer, and wherein the material of the first powder coating layer (e.g. the base coating layer) is a smooth coating layer material and the material of the second powder coating layer (e.g. the top coating layer) is a textured powder coating layer material. Preferably, the textured powder coating layer is a fine or micro-textured powder coating, most preferably a micro-textured powder coating. Both coatings of the present invention are based on low temperature curing powder compositions suitable for application to heat sensitive substrates, i.e. they cure at temperatures that do not damage heat sensitive substrates. Typically, this means that the powder coating composition is cured at a temperature below 150 ℃, preferably below 140 ℃, more preferably below 120 ℃, most preferably below 100 ℃.

In order to obtain the effect envisaged by the present invention, it is important that the first powder coating layer is malleable and has a non-hard surface, i.e. uncured or only partially cured, when the second powder coating layer is applied onto the first powder coating layer.

Accordingly, the present invention relates in a first aspect to a method of powder coating an article comprising:

a) providing a substrate coated with a first powder coating layer,

b) applying a second powder coating layer onto the first layer, the first layer being in an uncured or only partially cured state,

c) curing the first and second layers of the first and second layers,

wherein the material of the first powder coating layer is a smooth powder coating layer material and the material of the second powder coating layer is a textured powder coating layer material.

The invention also relates to a coated article comprising a heat-sensitive substrate and a powder coating comprising two layers, wherein the powder coating has a gloss value measured at 60 ° of less than 40 gloss units and an average roughness Sa of less than 1.50 μm, preferably less than 1.00 μm.

The first powder coating layer may already be in an uncured or partially cured state when applied to a surface. In this case, the process of the invention is the so-called dry-on-dry process, which is a well-known and conventional practice in powder coatings. Alternatively, the first powder coating layer is brought to an uncured or only partially cured state after application to the substrate and before application of the second powder coating layer by heating the first layer so as to at least partially melt or soften the first powder coating layer.

Thus according to one embodiment, the present invention relates to a method of powder coating an article comprising:

a) providing a substrate coated with a first powder coating layer,

b) heating the first layer to at least partially melt or soften the first powder coating layer such that said first powder coating layer is uncured or only partially cured,

c) applying a second powder coating layer onto the first layer,

d) curing the first and second layers of the first and second layers,

Without being bound by theory, the material of the second powder coating layer may at least partially sink into the material of the first powder coating layer, thereby reducing or minimizing the surface roughness of the top surface of the second layer. Thus in one aspect, the invention is based on the judicious insight that in embodiments, a smooth finish, and preferably an ultra-smooth finish, can be achieved by combining a powder coating composition with process steps in which a textured powder coating layer (e.g., a micro-textured powder coating layer) becomes (ultra) smooth when applied over a green cured ductile smooth powder basecoat layer. This enables the realization of finishes of the type having low gloss levels that cannot be achieved with the known powder coating methods, especially for heat-sensitive substrates.

The surface finish can be generally described as smooth, textured, or corrugated (the latter also referred to as wrinkles). A smooth surface finish may be defined as a surface that is substantially free of irregularities, roughness, or protrusions. The texture of the second coating may have varying roughness, which may be determined with a roughness meter that measures properties such as peak and valley lengths in the texture. Regardless of the texture/roughness of the second layer, by applying the method of the invention, the finish becomes flat and smooth (so-called satin-smooth).

The term "UV curable" includes the use of a coating composition curable with electromagnetic radiation having a wavelength in the range of 100 and 445nm, preferably provided by a radiation source having a specified frequency between 1.5PHz and 789 THz. More generally, actinic radiation may be used, and curing of the UV-curable coating layer refers to, for example, actinically initiated polymerization.

The term "ULB" (ultra low bake) curing powder coating composition is understood to be based on the powder coating type, which comprises the principle of thermally initiated or radiation initiated thermal curing polymerization.

In the prior art, for heat curable coatings, a 50-70% cure progression is sometimes referred to as the "green cure" stage. In this application, green body cure may refer to, for example, 0-70% cure, such as 10-70% or 50-70% cure.

The method according to the invention comprises the step of providing a substrate coated with a first powder coating layer. The substrate may for example comprise a heat sensitive substrate, such as a substrate susceptible to chemical and/or physical modification (e.g. deformation and/or discoloration) at the temperatures and times typically used in powder coating processes, such as at temperatures above 180 ℃, above 160 ℃ or above 140 ℃ for a period of at least 10 minutes, at least 5 minutes or at least 2 minutes. The substrate may, for example, comprise a lignocellulosic material. The substrate may for example comprise or consist of wood or a wood fibre based product such as MDF (medium density fibreboard), or any other type of fibreboard such as HDF (high density fibreboard). The substrate may for example comprise compressed wood fibres and/or particles and typically a binder resin. The substrate may thus for example also comprise particle board such as oriented strand board and plywood. The substrate may also comprise a plastics material such as a thermoplastic polymer, for example having a glass transition temperature and/or melting temperature of less than 200 ℃ or less than 180 ℃, or less than 160 ℃, or less than 140 ℃. The substrate may also comprise, for example, fiber cement, paper plaster, paper foil, or a plastic composite. In an interesting embodiment, the substrate is a composite material, for example a composite material comprising such a plastic material and a wood based material.

The coated articles can be used, for example, in residential buildings, commercial buildings, office partitions, joinery, ceilings, retail equipment, and marine equipment such as in ships. The coated article may be, for example, furniture, for example in assembled form or flat packaging. The coated article is, for example, a furniture component.

The powder coating composition of this embodiment is typically suitable for outdoor and/or indoor applications.

In order to allow proper powder coating deposition while electrostatically spraying, the resistance of the substrate, or at least the substrate surface, is typically below 10¹¹Ω。

The substrate may, for example, have a wide variety of two-dimensional and three-dimensional geometries such as plates or beams, boxes, baffles, frames, cut and connected (routed) components, or combinations thereof. The substrate has, for example, a flat or curved surface to be coated.

The substrate is coated with a first powder coating layer. In one embodiment, the substrate is provided as a substrate coated with such a first layer in a previous step. The method optionally may further comprise the step of coating the substrate with a first powder coating layer, and optionally a pre-treatment step. Pretreatment steps are, for example, the application of conductive primers, sealants, smoothing (e.g., thermal smoothing), and special deionized air brush cleaning. The substrate is optionally preheated to aid in uniform powder application. Optional preheating comprises heating, for example by one or more selected from infrared radiation, electrical heating, gaseous infrared heating and convection. The first powder coating layer is typically applied by electrostatic spraying.

The method may comprise the steps of: heating the first layer so as to at least partially melt or soften the first powder coating layer, wherein the first powder coating layer is uncured or only partially cured. Such as heating the substrate and the first layer. The heating step may for example comprise heating with convection and/or Infrared (IR) heating, advantageously a combination of both. In this step, the first powder coating composition typically melts, levels, and optionally partially cures. Typically to a temperature above the glass transition temperature (T) of the first powder coating composition_g) And/or above the melting temperature (T)_m) The temperature of (2). Tg is measured, for example, using differential scanning calorimetry according to, for example, ASTM E1356 as the midpoint of the temperature range bounded by the tangent to the flat region of the DSC heat flow (J/s) curve. Tm is, for example, the endothermic peak in the DSC heat flow curve, which is used to characterize melting. In DSC analysis, any exothermic value resulting from the residual reaction potential can likewise be detected. By starting at e.g. 40 ℃ in the first run, running a heating rate such as 5K/min until the desired curing temperature Tc (e.g. 145 ℃) is reached, the powder coating composition will be fully cured during the measurement. The amount of exothermic energy generated by the chemical reaction (mJ/g) can be read at the peak of the curve. The sample was cooled and run a second time at 5K/min for the same period until the exothermic reaction peak was no longer detectable, indicating that the system had reached full cure.

Any partial pre-solidification that may occur during the green body solidification melting process may be detected by calculating the difference between the fully solidified exotherm values and the measured exotherm values for the coating. The method allows for the determination and quantification of partial cure.

In the case of a heat-curable first powder composition, the heating is preferably to a temperature below the curing temperature, for example at least 5 ℃ below the curing temperature, and/or preferably for a short time above the curing temperature Tc, so that no curing or only partial curing is obtained before the application of the second powder coating layer. For example, the first layer may have been subjected to a temperature above (Tc-5 ℃) for less than 10 minutes or less than 2 minutes before the second layer of material is applied to the substrate. Preferably, the first coating layer forms a film, more preferably a homogeneous film, in this step. Preferably, the first powder coating at least partially melts and/or softens during this stage, for example to produce a liquefied and/or gelled layer. Preferably, the heating is to a temperature of at least 75 ℃, at least 95 ℃, or at least 120 ℃, or at least 135 ℃ for a period of at least 10 seconds, at least 30 seconds, at least 1 minute, or at least 5 minutes.

During this step, the first layer is uncured, or only partially cured. This results in a so-called "green cured" article. For example, if partial curing occurs in this step, the partial curing is less than 80%, less than 70%, less than 20%, or less than 10%, or less than 5% or less than 1% of full or final curing, e.g., as measured by the number of functional groups reacted. For example, the number average molecular weight of the first powder coating composition is less than 200% or less than 150% of the initial number average molecular weight while the second powder coating layer is applied. For ULB (ultra low bake) first layers, DSC (differential scanning calorimetry) can be used to obtain the exothermic value as an equivalent to any residual curing potential. For UV coatings not irradiated with UV light, the curing can be assumed to be 0% or in any case less than 10%.

The embodiment according to the invention in which the green cured first layer (base coat) is recoated with the second layer (top coat) provides good interlayer adhesion between the two layers. The quality of the interlayer adhesion can be measured, for example, with a cross-line test according to DIN EN ISO 2409. Results without delamination between the two layers can be obtained and indicate the best interlayer adhesion values.

The first layer preferably has a thickness of 10-200 μm, for example 50-90 μm. The powder composition is for example an Ultra Low Bake (ULB) or UV curable powder coating composition.

Advantageously, the method of the invention can avoid an abrasive surface treatment step, for example a sandblasting step. This is particularly advantageous for wood-based substrates, such as for substrates comprising wood fibers and for other types of fiber-containing substrates, which provide at least part of the surface to be coated. Thus, in a preferred embodiment, the substrate exhibits fiber stand-up prior to application of the coating. In the coated article, fiber set-up may be at least partially prevented by the coating. Also for such substrates, the method optionally does not include sandblasting.

Furthermore, advantageously no primer (sealant) is required and the first powder coating layer is preferably applied directly to the substrate. In this way a very efficient coating method is provided, which may advantageously comprise fewer steps compared to known powder coating methods (for e.g. heat sensitive substrates). In a preferred embodiment, the total processing time from application of the first coating layer to complete curing of the first and second layers is less than 30 minutes, less than 20 minutes, or less than 10 minutes.

The method of the invention comprises the step of applying a second powder coating layer (topcoat) onto the first layer (basecoat). The second powder coating layer is typically applied using electrostatic spraying, using triboelectric or high-voltage discharge spraying equipment such as a spray gun. The second layer is applied, for example, when the temperature of the first layer is 10-70 deg.C, or 15-50 deg.C, for example at ambient temperature such as 15-30 deg.C, or 18-25 deg.C, preferably at 40-45% relative humidity. The invention particularly relates to powder coating compositions applied as free-flowing dry powders. The powder coating compositions are typically applied solvent-free. This applies both to the application of the second layer and to the optional step of applying the first layer.

Between the heating stage (in particular melting of the base coat) and the step of applying the second coating layer, the part to be cured may for example be stored. Thus, the green cured part is optionally stored. This advantageously may allow the method to be performed with limited equipment, for example with a single spray chamber and/or a single oven.

Advantageously, in some embodiments, the remaining surface heat activation (e.g., higher temperature) from the heating step of the first layer enables the second layer electrostatic powder deposition. This may allow omitting the preheating unit for the second powder application step.

In a preferred embodiment, it is furthermore advantageous for the process to be carried out in a production line having separate runs for the first and second layer, wherein different line speeds can be used for these runs. This reduces the overall processing time and energy.

After the second layer is applied to the first layer and the substrate, the method further comprises the step of curing the first (base coat) and second (top coat) layers. Thus, for the substrate on which both the first and the second powder coating layers are applied, a complete curing is achieved. The curing step may comprise thermal curing, for example using a temperature of 100-. One possible curing cycle for a low bake or ultra low bake coating layer is, for example, 5-10 minutes at 130-135 ℃. The solidification phase includes, for example, melting, flowing and complete solidification of the two layers applied.

UV-curable powder coatings advantageously allow good separation of melting and curing, since for such compositions melting (and softening) is temperature dependent, whereas curing requires UV light. Thus, UV light is generally not applied or avoided in the step of heating to at least partially melt or soften the first powder coating layer. UV-curable powder coatings also allow low processing temperatures. Furthermore, UV crosslinking (i.e., curing) produces coatings that can have an advantageous combination of high crosslinking, high hardness, and chemical resistance.

One possible cycle for the UV powder coating according to the described embodiment of the invention comprises e.g. for the first (base) layer melting typically at a temperature below 120 ℃, below 100 ℃, below 85 ℃ or below 75 ℃ for 0.3-3 minutes to obtain a film which melts and forms on the substrate and subsequently for the second layer applied (top coating) melting typically at a temperature below 120 ℃, or below 100 ℃ or below 85 ℃ or below 75 ℃ for 0.3-10 minutes, more preferably 0.3-5 minutes, or more preferably 0.3-2 minutes and subsequent exposure to 100-.

In one embodiment of the present invention, a satin smooth surface aspect is provided. In one embodiment, the provided coating has a gloss value of less than 50GU, less than 40GU or less than 30 GU. Such low gloss values can be obtained by varying the powder coating used in the following respects: for example melt viscosity for the base coat and texture type used for the top coat. Gloss values can be measured with a gloss meter, such as according to ISO 2813:2014, where gloss values (expressed in GU-gloss units) at 60 ° angle geometry are applied.

In the present invention, the first layer (basecoat) is a smooth powder coating of a material preferably having a gloss value about at least 15GU higher, at least 30GU higher or even at least 50GU higher than the second layer (topcoat) powder coating applied as a single layer. The second layer (top coat) is a textured powder coating layer, wherein the gloss value is typically controlled by the type of texturing agent used in the second layer. Due to the combination of layers in the coating of the present invention, the actual gloss value of the second layer may be different from the gloss value specified for the powder coating formulation of the second layer (i.e., a separate layer without the first layer). This can be advantageously used to adjust the gloss value of the coating. The combination of a smooth powder coating first layer and a textured powder coating second layer is required to develop the satin smooth surface aspect, which is a preferred embodiment of the present invention.

The material of the first lubricious coating layer can, for example, include a flow agent (also referred to as a filler). Flow agents are rheology modifiers that reduce the melt viscosity of the powder coating formulation and reduce the surface tension of the melt, allowing it to flow and extend over a surface. Examples of suitable flow agents include, but are not limited to, polyacrylates adsorbed onto silica, amide modified polyether oligomers, and flow promoter masterbatches. These flow agents are typically used in amounts varying between 0.1 and 10 wt% based on the weight of the total powder coating formulation. Some coating formulations may not require the use of flow agents to achieve a smooth finish without defects (no craters, no fish eyes, etc.).

Current technology in formulating textured powder coatings requires balancing particle size, resin viscosity, and filler loading to limit the melt flow of the material, thereby resulting in a coating surface having various degrees of orange peel or texture. The material of the second textured coating layer may therefore comprise a texturing agent. The texturing agent acts as a rheology modifier, increasing the viscosity of the powder coating formulation and creating a shrinking effect on the powder coating, which results in a textured finish. Typically, the texturing effect may be achieved by using such texturing agents as Polytetrafluoroethylene (PTFE), polyethylene, a combination of both and/or fillers. Texturing agents typically comprise particles, for example particles having a particle size different from that of the powder coating. Such texturing agents typically consist of particles having a controlled classification of their particle size distribution. Furthermore, those agents may be based on organic or inorganic materials or combinations thereof and are characterized by a softening or melting point that is at least twice, preferably at least 1.5 times, the melting temperature range required for the powder coating process. Such texturing agents typically comprise a particle size distribution in the range of 1-250 μm, preferably in the class of 1-150 μm, more preferably in the class of 1-100 μm, even more preferably in the class of 1-50 μm or 1-25 μm. Typical particles have a spherical or anisotropic structure. Embedded in a film-forming binder matrix, provides an overcoat referred to as a textured or microtextured surface aspect. Typical amounts of texturing agents are from 0.1 to 30% by weight, based on the total weight of the powder coating formulation. Generally, higher texturing agent content will provide a rougher textured finish and lower gloss level. Other factors that affect the texture type are the shape of the texturing additive (spherical or anisotropic) and its classification within the particle size distribution range, the particle size of the powder, and the means of powder application such as voltage and amperage. Optionally, the second layer is a micro-textured powder composition.

The combination of the flow agent in the first layer and the one or more texturing agents in the second layer contributes to a smooth, low gloss finish. In some embodiments, the material of the second layer comprises a flow agent at a lower concentration (on a weight basis) than in the first layer, preferably with a flow agent concentration at least 50% lower than in the first layer. Preferably, the material of the first layer does not comprise a texturing agent, or at least comprises it in a lower concentration (on a weight basis) than in the second layer, preferably in a concentration of at least 50% lower.

The gloss of the finish is controlled by the gloss of the second textured powder coating layer. Typically, the rougher the textured powder coating, the lower the gloss and the matte the finish will be. The best results for the silk smooth finish are achieved when the second layer is a micro-textured powder composition.

Examples of heat curable powder coating compositions, such as ULB coating compositions, for the first and/or second coating layer include systems based on free radical initiated heat curable powder coating systems. Examples are unsaturated polyesters, vinyl ether-polyurethane polymers and peroxide compositions. It may also be a heat-curable resin composition such as a-COOH functional (carboxylic acid) saturated/unsaturated polyester resin having an epoxy group. They may for example be derived from bisphenols such as bisphenol a based epoxy resins, suitably having a Mn (number average molecular weight) of at least 1000, preferably 1000-4000. For example, the powder coating composition may be cured using thermal radicals, e.g. comprising a peroxide initiator.

Examples of actinic radiation curable, e.g. UV curable, powder coating compositions include systems containing binder unsaturated resins such as unsaturated (meth) acrylate resins, unsaturated allyl resins, unsaturated vinyl resins. But also for example acrylated epoxies, acrylated aliphatic or aromatic polyurethane oligomers, acrylated polyesters or acrylic oligomers, semi-crystalline or amorphous polyesters. The UV-curable composition preferably contains a photoinitiator. Optionally, the composition contains free radical initiators (e.g., peroxides and azobisisobutyronitrile), and additives such as flow agents, defoamers, wetting agents, slip aids, and other coating additives. It may also be a Polytetrafluoroethylene (PTFE) additive for improved robustness and scratch resistance, particularly for the second coating. Flow agents are also known in the art. Examples are polyacrylates, such as adsorbed on a silica support or together with a binder resin, and (cellulose) ester compounds.

Preferably, the material of the first layer (undercoat) comprises a powder coating formulation with a crystalline resin. Such crystalline resins often have a low viscosity of the molten resin, which is advantageous for ensuring that the material of the second layer can at least partially sink into the first layer during the coating process.

The first and/or second powder coating composition for example has a melt viscosity at 90 ℃ of less than 1200pa.s, or less than 1000pa.s, or even less than 800 pa.s. Such low melt viscosities are useful, for example, for formulating powder coating compositions for horizontal application.

The first and/or second powder coating composition may also have a melt viscosity at 90 ℃ of at least 1200pa.s, or at least 2000pa.s, or more preferably higher than 3500 pa.s. The powder composition may also have a melt viscosity at a melting temperature at 100 ℃ of at least 1000pa.s to 2500pa.s, or 900pa.s to 1800 pa.s. Such viscosities are used, for example, to formulate powder coating compositions that are used for vertical application. These viscosities can be measured according to ISO 2884-1, a rotational viscometer (preferred method) or a cone and plate viscometer (for the melt from the uncured powder).

The first and/or second powder coating compositions have a particle size of, for example, 2 μm to 100 μm, or at least 2 μm to 80 μm.

The first and/or second powder coating compositions optionally comprise a pigment. The coatings can, for example, be formulated as clear coatings without pigments, which are used, for example, in indoor and outdoor applications. The low gloss clear coating powder coating composition may, for example, be used to protect a substrate and/or to avoid unwanted specular reflection from a substrate. The coating may be formulated for indoor use and/or outdoor use.

Preferably, the second powder coating layer is UV curable, and wherein said curing step comprises irradiating the article with UV light to cure the second powder coating layer. Preferably, both the first and second powder coating layers are UV curable, and wherein the curing step comprises irradiating the article with UV light to cure the first and second powder coating layers, and wherein the first powder coating layer is not cured until after the second powder coating layer is applied, and typically melted and cured.

Advantageously, in order to obtain a satin smooth surface effect with optimal results, in particular a regular and uniform finish, the UV curing may be independent of the temperature of the UV-curable coating layer and thus of the melting temperature of the powder coating composition when exposed to UV radiation.

Advantageously, this method allows to produce a satin-smooth surface effect along the entire surface in a very uniform manner. In particular, the method can be operated independently of the radiation angle (i.e. the angle between the UV radiation source used for curing and the surface). Care is taken to achieve a minimum UV dose at which the UV powder coating composition is fully cured. This is particularly advantageous for substrates having curved, connected (routed) or angled surfaces to be coated.

The method also works independently of the type of UV radiation source used and the UV specific wavelength spectrum emitted and produces a satin smooth surface, provided that the total UV dose ensures complete curing of the coating.

Also for UV curable coatings such as a UV curable second layer, the coating process may comprise melting the powder coating composition, e.g. at a temperature of at least 50 ℃, at least 60 ℃, at least 70 ℃, or at least 80 ℃, e.g. 80-90 ℃. The melting and curing steps can be applied separately to the UV curable powder coating composition, especially since the temperature does not initiate crosslinking but rather facilitates melting and film formation. Preferably, the substrate with the melted UV-curable coating layer enters the UV irradiation zone at a temperature at which the free radical reactive groups of the polymer used have sufficient mobility to polymerize properly. Furthermore, the processing times advantageously used for UV curing are generally much shorter than for ULB powder coating layers.

Advantageously, the uniformity of the satin smooth surface effect does not depend on the layer thickness of the applied first layer nor on the film thickness of the applied second layer, as long as both the first and second layers each have an applied film thickness of at least preferably 40 μm, or more preferably 50 μm.

Preferably, the material of the second powder coating layer at least partially penetrates into said first layer when both are in molten form. Preferably, the second layer is applied thermally and cured in such a way that the first layer is soft at some stage of applying the second layer to cure the second layer. Preferably, the method allows a portion of the second layer to "sink" into the first layer.

Preferably, a mixed layer is formed, wherein the first and second layers are mixed, wherein the thickness of the mixed layer is at least 1.0 μm, at least 2.0 μm, at least 5 μm or at least 10 μm. In the hybrid layer, it is preferred that the fraction of the first layer in the layer by volume (measured, for example, as the area of the cross-section) is from 10 to 90%, preferably from 30 to 70%. The upper and lower limits of the mixed layer in terms of coating thickness are given by the layers from which the fraction of the first layer is greater than 90% (at the bottom) and less than 10% (at the top, thus going into the second layer), for example measured in a cross section of the coating thickness and within a section parallel to the substrate of at least 1mm or at least 2 mm. Preferably, a mixed layer is formed, wherein the first and second layers are mixed between two unmixed layers, wherein in the mixed layer the fraction of line segments through the material of the first layer is 10-90%, preferably 30-70%, for each line segment of at least 1mm parallel to the substrate, and wherein in each unmixed layer the fraction of such line segments through the material of the first powder coating layer is at least 90% for the bottom unmixed layer and less than 10% for the top unmixed layer. In these cases, the first and second powder coating compositions have different compositions.

In a preferred embodiment, the first layer is heated to a temperature below 140 ℃, preferably below 100 ℃, to melt the particles of the powder coating composition without curing the particles. In the curing step, the substrate comprising the first and second layers is heated to a temperature below 140 ℃, preferably below 100 ℃ or subjected to UV irradiation to obtain a coating having a smooth surface. Preferably, the gloss of the obtained coating is adjusted by means of the type of powder coating particles selected for the second layer. For example, the gloss and texture of the second layer may be adjusted. The gloss of the coating may be increased, for example, by using a finer powder for the top coat and/or a top coat material having a higher gloss value. For example, the top coat material may contain a PTFE additive, particularly a PTFE additive containing PTFE particles and a coating shell of the particles. In some embodiments, both the first and second layer materials are ULB powder coating compositions. In other embodiments, both the first and second layer materials are UV powder coating compositions.

In summary, a satin smooth surface aspect may be achieved in embodiments of the present invention, in particular with UV powder coating compositions. Preferably, the first layer or base coat (or primer) is "green cured", and the second layer (top coat) is applied, and the complete curing of both layers preferably produces a smooth low gloss finish, a so-called satin smooth surface aspect.

More preferably, such a smooth (e.g. low surface roughness) and low gloss finish is obtained for heat sensitive substrates, especially if both the first and second layers are UV curable, since the overall processing temperature can be run at much lower temperature values. Preferably, the first layer (i.e., the base coat) is smooth, has a low viscosity, and preferably the second layer (or the top coat) is textured. Preferably, the article so coated has no or low orange peel. The smallest orange peel indicates a smooth surface.

Advantageously, the method of the invention allows to avoid the use of solvents, high utilization of the paint materials, and simple coating methods. A significant reduction or even elimination of surface orange peel can be achieved compared to known UV powder coating methods.

Advantageously, the method of the present invention can eliminate any necessary pre-treatment of the substrate exhibiting a fiber-setting effect when the coating has been applied.

A low orange peel can be achieved compared to known ULB powder coating methods.

Furthermore, articles for indoor as well as for outdoor use may be coated with the method of the present invention.

Generally, one advantage over liquid coatings is that powder coatings can deliver high film thicknesses, e.g., 100 μm to 200 μm, in a single application, while still achieving good through cure. Furthermore, the powder coating allows for a preferred embodiment wherein in a single step the first and/or second powder coating layer is applied onto a surface, in particular onto a vertically oriented flat panel (with respect to gravity), more preferably onto both sides of such a panel. In addition, the method of the present invention allows for the application of powder coatings to horizontal surfaces such as panels.

In general, embodiments allow for coating heat-sensitive substrates with highly developed and sustainable application methods and with environmentally driven coating materials.

The coating process advantageously comprises curing at relatively low temperatures and is particularly suitable for MDF and wood. The process can produce a matte or satin surface depending on the choice of the type of powder coating composition used for the second coating layer. The second powder coating composition is suitably a textured coating composition so as to produce a low gloss surface. Preferably, the coating of the present invention is satin smooth and combines a low surface roughness with a low gloss. Advantageously, the powder coating obtained may have an extremely regular matte effect, especially if the first and/or second layer is UV-curable, e.g. the surface is uniformly matte, e.g. without any higher gloss portions or spots. The method can also provide regular matte with a matte surface that is advantageously consistent and repeatable.

Advantageously, the coating method of the invention can be applied independently of the layer thickness applied, when applied, for example for the UV-curable first and/or second coating layer and for the ULB first and/or second coating layer, preferably the first layer is 5-200 μm, more preferably 50-70 μm, and the second layer is preferably 5-200 μm, more preferably 50-100 μm thick.

Preferably, the coated article has a matte surface, e.g., a gloss of 30 gloss units or less, or 20 gloss units or less, e.g., 15 to 30 gloss units, measured at 60 ° using a gloss meter according to ISO 2813:2014 and/or DIN67530, the first method being preferred. The coated article may also, for example, have a semi-gloss finish having 30 to 50 gloss units measured at 60 °.

Preferably, a smooth surface aspect is obtained. Thus, when cured, the first and/or second powder coating compositions preferably have amorphous polymer characteristics.

Furthermore, the method advantageously allows for efficient, i.e. less, inventory of coating components by using separate compositions for the two layers. The color matching process between the fine-grained coating and the smooth coating is also simplified. Advantageously, when the product is required to change from smooth to fine-grained, the risk of cross-contamination in the production line can be reduced, due to the flexibility of the coating process, which allows to adjust the desired gloss. In particular, the same binder matrix may be used for the powder coating compositions of the first and second layers.

Furthermore, the present invention relates to a method of powder coating an article comprising: providing a substrate coated with a first powder coating layer; optionally heating the first layer to at least partially melt or soften the first powder coating layer such that said first powder coating layer is uncured or only partially cured; applying a second powder coating layer to the first layer that is uncured or only partially cured; curing the first and second layers, wherein the material of the first powder coating layer comprises a flow agent and the material of the second powder coating layer comprises a texturing agent.

The invention also provides a coated article, preferably obtained with said method. The coated article preferably comprises a heat-sensitive substrate and comprises a powder coating comprising two layers. The powder coating preferably has a low gloss and a smooth surface. Preferably, the coated article has said satin smooth surface aspect. For example, the powder coating has a low gloss measured at 60 ° of less than 40 gloss units, or less than 30 gloss units, or less than 20 gloss units. Preferably, the surface is smooth, and thus the surface roughness is relatively low. For example, coated articles can be prepared which have a Sa (roughness average) of less than 1.50 μm or less than 1.20 μm or even less than 1.00 μm, and for example greater than 0.50 μm, and preferably have a Sq (root mean square roughness) of less than 1.50 μm, less than 1.30 μm or less than 1.20 μm, and for example greater than 0.50 μm. Sa and Sq are measured according to ISO25178 using, for example, wide field confocal microscopy. Alternatively and/or in combination, the coating has an Ra of less than 5.0 μm, or less than 2 μm, or less than 1.50 μm. Preferably, the coating has an Rz of less than 10 μm, or less than 6.0 μm. Preferably, the coating has an Rq of less than 5.0 μm or less than 2 μm. Where Ra is the average roughness; rz is the ISO ten point height measurement and Rq is the root mean square roughness, all measured according to ISO 4287 and, for example, ISO 4288.

The two layers are in interfacial contact, i.e. have an interface between them. The coating comprises a mixed layer, preferably as described above. The thickness of the mixed layer is at least 1.0 μm, at least 2.0 μm, at least 5 μm, or at least 10 μm. Preferably, the interface is corrugated such that in cross-section the length of the interface line is at least 1.1 times, or at least 1.2 times the corresponding length of the top surface in said cross-section of the total coating thickness.

The substrate is preferably as described above, and more preferably comprises MDF.

Drawings

The invention will be further described with reference to the accompanying drawings.

Fig. 1 shows, from left to right, a) an MDF part coated with a reference micro-textured powder coating, B) a part coated according to an exemplary embodiment of the invention, and C) a reference coated part with a high gloss coating. The substrate is the same MDF material in parts a-C. UV powder coatings were used in a and B and different liquid coatings were used in C to show a high gloss smooth finish.

In the coated element B of the invention, the composition of the top coat is the same as in the reference element a. However, in element a the coating is applied directly to the substrate, whereas in coated element B the coating is applied in a UV-curable manner with a primer powder coating in a two-step process according to the invention;

fig. 2 shows 235-fold magnification cut edges of a reference micro-textured coating (a) and a coating of the invention (B) on an MDF board. Part B shows, from bottom to top, an MDF board having a first layer and a red second layer. The top surface of B is flatter than A;

fig. 3 shows a 235-fold magnification of a surface with unpolarized light, where reflection is caused by peaks and valleys of the texture pattern and other irregularities of the surface. Thus, fewer bright spots indicate a smoother finish. A is low gloss, B is according to the invention, C is a high gloss coating of reference, having the same coated parts as in fig. 1. This shows that part B has a higher smoothness than part a. Part C showed sandblasted and polished scratches;

fig. 4 shows a) a reference microtextured UV-curable coating on a primer MDF and B) destructive film thickness drilling of the inventive coating. Panel a shows a layer from a liquid heat stable primer, a UV cured micro textured coating, and a black marker from the center of the MDF outward. Panel B shows UV white primer, UV red micro-textured coating, and black marker pen from the center of the MDF outward. In fig. 4A, the primer layer is not softened upon the top coat process and thus the final finish is microtextured. In fig. 4B, the second red layer appears to melt in the first white layer;

fig. 5 shows the surface measurements of article a prepared in the working example.

Fig. 6 shows a surface image of a comparative coated article B.

Fig. 7 shows a surface image of article C prepared in the working example.

Fig. 8 shows a surface image of an article D manufactured in the working example.

Some exemplary embodiments of the invention will now be further illustrated by the following non-limiting examples.

Example 1

Coated article a was prepared according to the present invention and comparative coated article B had a single layer of a fine-grained coating. The powder composition for B was the same as the second top coat of a. The results are shown in table 1. The roughness of the coated article a was about 30% of the comparative coated article B.

Example 2

Coated article a is prepared according to the present invention using a first layer and a second layer both of which are UV curable. Comparative coated article B was also prepared with a single layer of microtextured UV curable powder coating. The powder composition for B was the same as for the second top coat of a. Surface texture properties were measured using wide field confocal microscopy using a Carl Zeiss Smartproof 5 confocal microscope with Zeiss EffectiontNavigation (ZEN) software. An area of 100mm x 100mm is defined on the surface and 5 areas of 4 x 4mm of the area are scanned. 16 individual sub-fields are defined in the 4 x 4mm region, with the lowest valley level and the highest peak level defined for each 1mm2 sub-field. In these vertical levels, maps of, for example, 1 μm vertical (z-axis) spacing were made, and 3D profiles were obtained.

Fig. 5 shows the surface measurement of article a. Fig. 6 shows a surface image of a comparative coated article B. The measured surface properties are given in table 2.

Coated article C was also prepared according to the invention using ULB powder coating with first and second layers. Fig. 7 shows a surface image of article C. A comparative coated article D was also prepared utilizing the same single layer microtextured powder coating composition as the second layer of article C. Fig. 8 shows a surface image of article D. Table 2 indicates the surface properties of the coated article C and the comparative coated article D.

Example 3

The example formulations are as follows:

formulation 1

Formulation 2

Formulation 3

Claims

1. A method of powder coating an article comprising:

a) providing a substrate coated with a first powder coating layer,

c) curing the first and second layers of the first and second layers,

2. The method according to claim 1, wherein between step a) and step b) an additional step is applied: heating the first layer to at least partially melt or soften the first powder coating layer such that said first powder coating layer is uncured or only partially cured.

3. The method according to claim 1 or 2, wherein the first powder coating layer and the second powder coating layer are based on a low temperature curing powder coating composition.

4. A method according to any one of the preceding claims, wherein the material of the second powder coating layer is UV curable, and wherein said curing step comprises irradiating the article with UV light to cure the second powder coating layer.

5. The method according to claim 4, wherein the material of both the first and second powder coating layers is UV curable, and wherein said curing step comprises irradiating the article with UV light to cure the first and second powder coating layers, and wherein the first powder coating layer is uncured until after the second powder coating layer is applied.

6. A method according to any one of claims 1 to 3, wherein the material of both the first and second powder coatings is an ultra low bake powder coating material.

7. The method according to any of the preceding claims, wherein the material of the second powder coating layer at least partially penetrates into the first layer.

8. The method according to claim 2, wherein in said further step the first layer is heated to a temperature below 140 ℃, preferably below 100 ℃, to melt the first powder coating composition without curing the first powder coating composition, and wherein in step c) the substrate comprising the first and second layers is heated to a temperature below 140 ℃, preferably below 100 ℃ or subjected to UV irradiation.

9. The method according to claim 8, wherein the surface of the coating has a gloss value measured at 60 ° of less than 40 gloss units and a mean roughness Sa of less than 1.50 μm.

10. The method according to any of the preceding claims, wherein the gloss of the obtained coating is adjusted by means of selecting the type of powder coating composition used in the second layer.

11. A method according to any preceding claim, wherein the material of the first layer comprises a flow agent and wherein the material of the second layer comprises a texturing agent.

12. A coated article comprising a heat-sensitive substrate and a powder coating comprising two layers, wherein the powder coating has a gloss value measured at 60 ° of less than 40 gloss units, and an average roughness Sa of less than 1.50 μ ι η, preferably less than 1.00 μ ι η.

13. The coated article according to claim 12, wherein the substrate comprises Medium Density Fiberboard (MDF).

14. A coated article according to claim 12 or 13, wherein the two layers are in interfacial contact and the coating comprises a mixed layer of the two layers having a thickness of at least 1 μm.